Multilayer structures are typically formed from a core sheet sandwiched between two face sheets. The core sheet of a multilayer structure defines substantial void space and has an effective cross-sectional thickness that is substantially greater than the cross-sectional thicknesses of the adjacent face sheets. Therefore, multilayer structures typically possess relatively high strength and stiffness at relatively low weight. As such, multilayer structures are used in various aerospace applications.
Superplastic forming is a known technique for manufacturing multilayer structures, such as expanded two-sheet panels (no core sheet) and expanded three-sheet panels (a core sheet positioned between two face sheets), which may be used as alternatives to traditional honeycomb multilayer structures. Superplastic forming is a metal forming process that takes advantage of the superplasticity of certain materials, such as titanium alloys, aluminum alloys and nickel alloys, at elevated temperatures. When such materials are heated to a superplastic state, they become pliable and can be expanded (e.g., by gas pressure) against a mold to achieve a desired shape. During expansion, the material can experience elongation of several hundred percent.
While the void space in multilayer structures advantageously reduces density (increases bulk), it presents a complication when a multilayer structure is subjected to thermally induced stress due to thermal gradients in the multilayer structure. For example, the multilayer structure may comprise a three-sheet panel (i.e., a core sheet positioned between two face sheets) that is used in a thrust reverser of an aircraft engine. In this example implementation, one face sheet (i.e., the inner sheet) faces toward the aircraft engine, and the other face sheet (i.e., the outer sheet) on the other side of the core sheet faces away from the aircraft engine. Since the inner sheet is closer to the aircraft engine than the outer sheet, the inner sheet is exposed to higher temperatures than the outer sheet during operation of the aircraft engine. The thermal gradient between the inner sheet and the outer sheet results in thermally induced stress in the three-sheet panel.
Accordingly, those skilled in the art continue with research and development efforts in the field of multilayer structures including three-sheet panels.